1. Field of the Invention
This invention relates to reflector antennas. More particularly, the invention provides improvements in reflector antenna pattern control, return loss performance and manufacturing cost efficiencies via a self supported sub reflector and feed tube assembly which may be formed by injection molding.
2. Description of Related Art
Many broadcast and or communications systems require antennas with a highly directional signal reception and or transmission characteristic. Reflector antennas focus a signal received by a dish shaped reflector upon a centrally mounted receiver. Alternatively, a sub reflector mounted at the focus point of the dish directs the signal into a wave guide and there through to the receiver. Because the dish shaped reflector only focuses a signal received from a single direction upon the receiver or sub reflector, reflector antennas are highly directional. When the reflector antenna is used to transmit a signal, the signals travel in reverse, also with high directivity.
Reflector antennas with a sub reflector supported and fed by a waveguide are relatively cost efficient and allow, for example, location of the transmitter and or receiver in an easily accessible location on the back of the reflector. This configuration eliminates the need for a support structure that spans the face of the reflector, partially blocking the reflector, and signal losses associated with passing the signal through a cable routed along the support structure. A waveguide with a generally circular or elliptical cross section provides the antenna with dual polarization capability.
Electrical performance of dual polarized reflector antennas with a self supported feed are typically measured with respect to gain, cross polarization, edge illumination and return loss characteristics.
Cross polarization is a form of interference that occurs when dual signals having different polarizations are simultaneously transmitted and or received by the antenna. Either of the dual signals may propagate on or reflect from surfaces of the sub reflector and/or waveguide partially transforming into the polarization mode of the other signal, creating inter-signal interference. To minimize cross polarization, prior self supported feed reflector antennas have applied corrugations to the sub reflector and/or waveguide, for example, as described in U.S. Pat. No. 4,963,878 issued Oct. 16, 1990 to Kildal.
Edge illumination refers to side lobes of the reflector antenna radiation pattern that degrade antenna directivity. A shroud lined with energy absorbing material may be added to the antenna to reduce edge illumination. However, a shroud only blocks and or absorbs edge illumination occurring at angles that intersect with the shroud. Also, shrouds increase the overall weight, wind load, structural support and manufacturing costs of the antenna. An alternative method of reducing edge illumination is use of a “deep” reflector dish and the addition of corrugations proximate the outer radius of the sub reflector to inhibit surface propagation and or field diffraction around the outer edge of the sub reflector as described in U.S. Pat. No. 5,959,590 issued Sep. 28, 1999 to Sanford et al.
Return loss is a measure of the portion of signal that, rather than being projected forward from the reflector, is returned to the transmitter. Sources of return loss in a self supported feed include the sub reflector surfaces, impedance discontinuities in the waveguide, secondary reflection from the vertex area of main reflector and or in the attachment structure between the waveguide and the sub reflector.
In both U.S. Pat. Nos. 4,963,878 and 5,959,590, the sub reflector is attached to the waveguide by a dielectric block that positions the sub reflector at a desired orientation and distance from the end of the waveguide. The interfaces between the dielectric block, waveguide, sub reflector and any adhesives or mechanical interlocks used to secure the components together create impedance discontinuities that are significant sources of return loss.
U.S. Pat. No. 6,107,973 issued Aug. 22, 2000 to Knop et al., assigned to Andrew Corporation as is the present invention, describes a reflector antenna with a self supported feed using a profiled sub reflector and a shroud. A hollow dielectric cone coupled at the narrow end to a metal waveguide and at the wide end to a metal sub reflector orients and retains the sub reflector with respect to the end of the waveguide. The thickness of the cone sidewall dielectric material, thin in comparison to the dielectric blocks of the prior patents described above, is selected to create a phase canceling effect between the signal passing through the material and the signal reflected by the dielectric material. The features of the sub reflector, waveguide, hollow dielectric cone and the precision threaded mating surfaces between each of them are relatively complex and therefore expensive to manufacture. A plurality of seals are used between each of the separate components comprising the feed assembly, each representing a possible moisture penetration point should the seal(s) fail over time. Also, an additional hub component is required to mount the self supported feed to the reflector
Competition within the reflector antenna industry has focused attention on antenna designs that reduce antenna materials and manufacturing costs but which still satisfy and or improve upon stringent electrical specifications,
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.